Interesting enough I have all three so I can share my experience. If you are a gamer like myself I believe you should pick the 5.1.
Dual Monitors
I was so excited to get this setup I nearly ripped the boxes open with my claws. My sister was amused and said it had look like big foot attacked the box. In any case, I could not be more dissapointed. As a gamer, my second monitor is usually off or on standby. I am not saying the second monitor is not useful. Every once in a while when I am trying to read and write (i.e. reading directions for one thing while doing it on the other screen [like setting up pop for gmail, etc..]) is when it becomes useful. IMO, this is almost as pointless as Dual Cores. To answer your question, a 17" LCD and 19" LCD both have the native resolution of 1280x1024, so it doesn't really matter.
Dual Core
I recently bought myself an Opteron 165. This chip is badass, but as you can imagine it has very little effect for games. I have not had a chance to use it yet when I have to encode a dvd or doing something else that is CPU intensive while playing a game. It is awesome bragging rights, and maybe is better for gaming in the sense that all of your background programs can run off of the second core while you are gaming on the first. Still did not seem worth the investment to me.
Klipsch Promedia 5.1 Ultras
Paired with a decent sound card (i.e. not onboard sound), this single handedly increased my gaming experience 10 fold. Assuming you have an already decent gaming rig, this is the ultimate upgrade. It really gives you a sense of where the world is around you.